Hint

Answer

The damp air weighs less because water molecules (molecular weight of 18) have replaced more massive molecules of air (nitrogen: molecular weight of 28, oxygen: molecular weight of 32). Both volumes have the same number of molecules. Therefore, a humidity increase lowers the barometer reading. The barometer pressure value gets lower as a storm approaches. (This question is from "Mad About Physics", a must have book for all science types.)Hide

Comments

1 problem. The definition of weight. Weight is mass * gravity. mass of air is less than water. essentially, gravity is cancelled and your left comparing average masses. Mass of water + air is more than just water so the damp air "weighs" more.

Your logic is sound but your facts are wrong. Please read my answer again. It is correct. I am not making this one up, it is based on known science and taken straight out of a physics book. If you were correct then a humidity increase would raise the barometric pressure, but reality tells us that it lowers it.

To big Papa the only time mass and weight need to be considered is in a situation where grvitational attraction varies such as on another planet or going up a mountain. If two things are weighed with no variation to gravity (as in this case) there is no need to make a distinction between mass and weight.

I want to comment on the pressure reading when a storm approaches. Even though the problem is correct in saying that damp air weighs less, the part about the storm is not. Low pressure causes the storm, not the storm causes the low pressure. Storms are cyclones. They are areas of low pressure. Since the area around it is at high pressure, winds form and blow everything toward the center of the cyclone. Once the air reaches the center, the only place it can go is up. When this happens, the air cools and any vapor condenses and forms clouds. That is why storms have lower pressure.

The opposite are anticyclones, which result in fair weather. Anticylones are areas of high pressure and thus air is blown outward toward areas of lower pressure. This will result in air being drawn from the upper atmosphere. This air will warm and any condensate in the air will vaporize. Thus high pressure regions have clear skies.

Good teaser, and great explanation! I sort of knew that the water molecule had to be lighter than the dry air molecule because of the hydrogen, but didn't know the molecular weights! Again, great teaser!